1. Field of the Invention
This invention relates to compacts obtained from nonmetallic and metallic powders through die pressing. In particular, the invention relates to an improved method of manufacturing parts from powders where the properties of powders and green compacts can be readily controlled and modified to obtain final products according to desired specifications and dimensions.
2. Discussion of the Prior Art
Powder pressing consists of compacting dry loose powder in a rigid die at sufficiently high pressures so that a dense and strong piece is formed. Depending on the mix used and production life required, dies are made from hardened steel, abrasion resistant alloys, tungsten carbide or others. Automatic dry pressing of ceramics has been practiced since 1904. The technology, originally developed for steatite porcelains that are soft, flow easily and cause little die wear, has been adopted for many kinds of ceramics and powder metallurgy. Components made from powdered materials are typically produced to final shapes, without or with very little machining required. Such products achieve high tolerances, good surface finish and uniformity in shape.
The main drawback of a die pressing is nonuniform density distribution throughout the compact. The friction between a powder and a die's wall and between all individual powder particles creates a diffusion of pressure during compaction and, as a result, variations in density. On the other hand, high quality of a final product requires the density in a compact to be as uniform as possible. The factors affecting the density distributions are: the type of compacting technique, the type of tools used, and the properties of a powder to be pressed. Therefore, for a given compacting technique and a given tool design and material, the density distribution throughout a compact depends only on powder properties.
Among the most important powder parameters are: flow rate of a loose powder, bulk density (a packing characteristic of powder grains), friction coefficient between a powder and a die's wall, and compactibility coefficient.
The flow rate and bulk density are used to estimate the efficiency of a compacting process which is restricted by the time interval needed to fill up a die with a powder in an automated process. The coefficient of friction is of crucial importance to the technical side of the compacting process. It depends on the powder properties, the material of the die, and the quality of the die surfaces. Its magnitude characterizes the powder's capability for uniform densification along a height of a compact. The compactibility coefficient is assumed to depend on powder properties (such as interparticle friction) and may be treated as a measure of the powder capability to be compacted.
At the present time there are commonly used methods for determining the bulk density and the flow rate of powders but no reliable method exists for determining the friction coefficient and the compactibility coefficient. Many studies have been carried out to express in a mathematical form the distribution of the pressing forces in dies as a function of related properties and relationships during the compacting process. The most interesting relationship derived for one-end pressing of cylindrical samples is given by Ballhausen. It can be generalized for arbitrary samples of constant cross-section as, EQU p.sub.c /p.sub.d =exp (u.multidot.tan .phi..multidot.SH/F) (1)
Here,
p.sub.c --pressure applied to the top punch, PA1 p.sub.d --pressure transmitted to the bottom punch, PA1 u--friction coefficient PA1 .phi.--angle of a pressure transmission from the top punch to the die's wall, PA1 S--perimeter of the cross-section of the sample, PA1 H--height of the sample. PA1 p.sub.c --pressure applied to the top punch, PA1 p.sub.d --pressure transmitted to the bottom punch, PA1 S--perimeter of the cross-section of the sample, PA1 H--height of the sample. PA1 .eta.--slide coefficient. PA1 .rho..sub.rp --relative bulk density of a powder. PA1 P.sub.f --friction force between a compact and a die's wall, PA1 F.sub.1 --lateral area of a compact, PA1 W.sub.s --compression strength of a compact.
The magnitudes of p.sub.c, p.sub.d, S, H, and F in (1) can be easily measured. In order to determine the friction coefficient, u, it is necessary to correctly estimate the angle .phi. of the pressure transmission from the top punch to the die's walls. As yet, no reliable method has been proposed for determination of .phi. and thus an accurate friction coefficient u cannot be obtained. As seen in the relation (1), the magnitude of the friction coefficient can vary over a wide range.
A similar approach to the above is used by Gasiorek (see Gasiorek et al.) who gives the following empirically established relationship, EQU p.sub.d /p.sub.c =.eta..sup.SH/4F ( 2)
Here,
The slide coefficient .eta. characterizes interactions between the powder and the die's walls. For a given material and surface conditions of a die, the coefficient .eta. is a constant and describes the powder's ability to be uniformly densified during compaction. Its numerical value can vary in the range 0&lt;.eta.&lt;1.
The relation (2) allows one to calculate the slide coefficient and is of great practical importance. Comparing equations (1) and (2) it is seen that the slide coefficient and the friction coefficient are related, that is, EQU .eta.=exp (-4.multidot.u.multidot.tan .phi.) (3)
Numerous experiments (see Gasiorek et al.) with various cylindrical samples subjected to a wide range of technically applicable pressures have shown a great constancy of the slide coefficient consistent with relation (2). Therefore, the relation (2) can be rewritten as, EQU p.sub.dh =p.sub.c .eta..sup.Sh/4F ( 4)
where p.sub.dh is the pressure in a sample at a distance h from the face of the top punch. This relation allows to predict the pressure at any particular cross-section of the pressed compact once the sliding coefficient is known.
The non-uniform pressure distribution (4) creates non-homogeneous density of the compact along the direction of pressing. Despite the great technical importance of the density-pressure dependence, not much attention has been devoted to that aspect with relatively few papers reporting mainly results of direct density measurements (see Van Grenou). Such measurements are subjected to considerable errors that preclude any generalized description of the phenomenons accompanying the compaction process.
It has been determined (see Gasiorek et al.) that the density distribution along the height of the compact is linear. Thus, the apparent density measured at the half height of the compact, h=H/2, is equal to an average apparent density of the whole compact. The pressure p.sub.r at that height, called a reduced pressure, can be found from (4) to be: EQU p.sub.r =p.sub.c .eta..sup.SH/8F ( 5)
An introduction of the reduced pressure concept, p.sub.r, allows one to determine the compaction characteristic which does not depend on the SH/4F quantity. In addition, it could be shown that the above compaction characteristic for die pressing is identical as the compaction characteristic obtained for isostatic pressing process.
An extensive experimental investigation has indicated a logaritmic functional dependence between the reduced pressure and the density, EQU .vertline.log.vertline.log .rho..sub.ra .vertline..vertline.=f(log p.sub.r)(6)
where 92 .sub.ra is an average relative density of a given sample. Furthermore, the functional relation, f, has been determined to be linear, ##EQU1## Here, .alpha.--compactibility coefficient, p.sub.o --gravitational pressure of a powder,
The expression (7) can be transformed to ##EQU2## Substituting in (8) for p.sub.r the pressure at a given distance h from the face of the top punch given in (4), one gets an equation for density distribution along the height of a given sample, that is: ##EQU3##
The direct objective of pressing is to produce from a loose powder an agglomerate body having a definite shape and strength that will preserve itself during ejection from a die, transportation and other technological operations prior to sintering, and during sintering itself. Generally, the whole set of parameters defining the mechanical strength of a compact is called a cohesiveness of an agglomerate.
The cohesiveness of a compact for transportation and other handling purposes can be estimated by various comparative tests such as tumbling methods or impact resistivity measurements. In automatic pressing, the major problem is associated with the appearance of cracks in compacts. It has been commonly observed that the majority of cracks found in compacts before and after sintering is created during the ejection of the compact from the die.
It has been established experimentally (see Gasiorek et al.) that the cracks developed during the ejection stage can be avoided if, for one-end pressing, the following inequality is satisfied: EQU P.sub.f /F.sub.1 &lt;W.sub.s ( 10)
where
The friction force is given by EQU P.sub.f =P.sub.c .multidot.(1-.eta..sup.SH/4F) (11)
where P.sub.c is the total force applied to the top punch.
As described above, for a given powder and a given die the knowledge of the following parameters: slide coefficient, .eta., compactibility coefficient, .alpha., and cohesiveness, C, in addition to others easily measurable data is sufficient to determine major powder characteristics necessary for proper design of the compacting process to ensure most uniform density distribution and crack-free, highly accurate final product. The test procedure for obtaining the desired parameters is the subject of this invention.